The use of laser diodes for information storage and retrieval and for fiber optics applications is rapidly increasing. The compact size, brightness and switching rate of a laser diode are its greatest advantages.
A DC-to-DC converter power supply for a helium neon gas laser which is coupled directly to the laser is shown in U.S. Pat. No. 4,092,710, issued May 30, 1978 to Gary Earl Wadsworth, entitled "Laser Power Supply." The converter includes a power transistor which is subject to control of its on-time to provide constant current regulation.
U.S. Pat. No. 5,157,676, issued Oct. 20, 1992 to Russell B. Wilcox, which is entitled "Apparatus and Process for Active Pulse Intensity Control of Laser Beam," is another control scheme for a pulsed laser source in which the light beam from the laser is passed to a beamsplitter. A portion of the beam is tapped-off from the beamsplitter and supplied to a photo-detector circuit, which controls the output of the power supply. The power supply is coupled to a light intensity control device which is responsive to the voltage from the power supply. The beam from the pulse laser source passes through the light intensity control device in accordance with the regulation supplied by the photo-detector.
U.S. Pat. No. 4,369,525, issued Jan. 18, 1983 to Jean P. Breton, et al., entitled "Device for Automatic Regulation of the Output Power of a Transmitter Module in an Optical-Fiber Transmission System," discloses a device for automatically regulating the output power of a transmitter module in an optical-fiber transmission system. In this system a laser diode is positioned so that a portion of its output is coupled to a photodiode for control purposes. Current through the laser diode in this system consists of a direct or nominal current and a modulating current. A separate temperature sensor is included in the system for sensing temperature changes for controlling the nominal current.
The prior art regulation and control circuits have relatively low efficiencies. Switching power supply for laser diodes can improve the system electrical efficiency, particularly at high temperature. Until recently, however, switching voltage regulators were relatively too large in size for many applications. Recently, integrated switching regulator circuits in small integrated circuit packages have become available which open many new possibilities for the use of laser diodes. One such switching voltage regulator was announced by National Semiconductor in a brochure with a publication date of September 1992. National Semiconductor offered this switching regulator under the designations LM2574/LM2574HV under the trademark "Simple Switcher." This device is capable of driving a 0.5 amp load with output voltages of 3.3 volts, 5 volts, 12 volts, 15 volts and with an adjustable output voltage.
The switching voltage regulator is supplied in two types of packages, an 8-lead DIP package and a 14-lead surface mount package. In both packages 6 leads are utilizable for operation and control of the regulator. The remaining leads have no internal connection, but are soldered to a PC board to improve heat transfer. With these leads soldered to the printed circuit board, the copper traces on the printed circuit board are normally the only sinking that is needed because of the high efficiency of the regulator. In addition, small external components are required, and the regulator, as is common with switching regulators, includes internal frequency compensation. The active pins include a voltage input pin and voltage output pin, a separate signal and power ground pin, an on/off control pin for activating and deactivating the regulator and a feedback pin for supplying a feedback control signal to the regulator. National Semiconductor's Simple Switcher voltage regulator is only one of a number of regulators which may be employed to implement the present invention.
A fixed voltage version of the National Semiconductor voltage regulator is taught by the September 1992 publication. This version is illustrated in FIG. 1. Unregulated DC input voltage is supplied across the input capacitance .sub.in to pin 5 of the voltage or V.sub.in pin. This pin is coupled to an internal regulator which in turn is coupled to an on/off switch for external switching of the power supply off and on. In the 6-voltage version found in the publication, pin 3 is grounded to the same ground as the signal ground of pin 2. A separate power ground pin 4 is provided which may be coupled to the same ground as the signal ground pin 2. The output of the circuit is supplied on the pin 7. Suitable elements, such as a Schottky diode D1, the series inductance L1 and the output capacitance C.sub.out are coupled to pin 7 to provide the final regulated output to 0.5 amp load. capacitance C.sub.out and L1 to provide a feedback control
In the fixed voltage version, the feedback terminal pin 1 is coupled to the junction of the signal regulating the output current through the load. The feedback pin 1 is connected to an internal resistor which in turn is connected to an internal resistor R1 which is equal to a reference voltage at the other end. The junction point of the resistors R2 and R1 is coupled to the non-inverting input of a fixed gain error amplifier. The other input of the error amplifier is coupled to a 1.23 volt band gap reference level. The output of a fixed gain amplifier is connected to the non-inverting input of a comparator. The inverting comparator is connected to a 52 kHz oscillator.
The output of the comparator is connected to an OR gate the other input of which is connected to a reset circuit. The output of a NOR gate is connected to a driver which has current limiting and thermal shut-down control. The output of the driver is connected to a NPN emitter-coupled transistor a collector of which is coupled to receive the voltage input signal from pin 5, and the emitter of which is coupled to supply the output current to pin 7. Utilization of this fixed voltage version as shown with the laser diode as the load does not provide a desirable implementation. This fixing of the voltage across the laser diode does not provide satisfactory control of the output of the diode when temperature variations occur which is characteristic of laser diodes.
FIG. 2 is an adjustable voltage version, as taught by the September 1992 National Semiconductor publication. In this version the resistors R3 and R4 are added with R3 coupled to the load and R4 connected to ground, and the junction point between R3 and R4 is supplying the take-off point for the feedback that is supplied to pin 1. Variation in the ratio between R3 and R4 determines the amount of voltage that is fed back. With this implementation, the input voltage may undergo a wide variation. However, utilization of a laser diode that has a load that conflicts with this circuit again is not satisfactory since adjusting the output voltage is not determined by the output of the laser diode, but is determined by the ratio of the resistors R3 and R4.